In the case of Joss Whedon, it was boredom, not necessity, that proved the mother of invention. While he was on strike with fellow Hollywood screenwriters earlier this year, the creator of TV dramas such as Buffy the Vampire Slayer decided there had to be 'something more effective than walking up and down in a line with a placard'. So Whedon rounded up a few friends and set about making something quite unlike anything the world has seen before.

The result is Doctor Horrible's Sing-A-Long Blog - found at drhorrible.com - and it has been an instant summer success that is uniquely difficult to categorise. It is a TV show that has never been shown on TV, an internet video that looks nothing like an internet video. It was made for the fun of it, not for ratings or profit, and put on the web in the hope that someone would notice. The fact that they did, in their millions, has delivered a shock to the Hollywood system, raising the spectre of writers, directors and actors bypassing major producers and selling directly to audiences online.

'I was told several times that it's impossible,' Whedon, speaking from Los Angeles, told The Observer. 'But you can to an extent write your own rules. I realised there was nothing I wanted to do more, and the only person going to finance something as strange as this was me.'

Funny and fantastical, the musical adventure features aspiring super-villain Dr Horrible, who is hoping to defeat his nemesis, the arrogant Captain Hammer, win his dream girl and join the Evil League of Evil. Bad at being bad, his ray guns misfire and he inadvertently blows his cover by blogging his dastardly plots. The show, which first aired just over a fortnight ago, consists of three parts, runs for about 40 minutes and is neither a YouTube clip nor a movie. As Whedon noted: 'It's not "your cat falls off a TV set" or Ben-Hur - there is something in the middle.'

Dr Horrible owes its existence to the strike which paralysed Hollywood from last November to February this year, a dispute that partly concerned how much writers of TV series should earn when the studios put their content online. With other work on hold, Whedon's creative response to the issue was to sidestep the studios and come up with a show of his own. He got help from his brothers, Jed and Zack, and friends in the industry.

Whedon, 44, recalled: 'The show was written by our id. It came partly out of love and "We are the goofiest people about and say it loud". That joy gave us a sense of freedom. We had no restrictions on length or budget except what I was willing to spend - how much can I throw at this midlife crisis?'

The immediate cult hit status of the finished product appears to have exploded some myths about online viewing habits. 'Everyone has their theory about how long something can be on the internet. I know that if it's good enough to hold people's interest, it doesn't matter how long it is.' When the show was made freely available last month the site generated 200,000 hits per hour and soon crashed. Whedon added: 'First, part of this was just making an internet event. Second, if you can make money from this, you really have a business model.'

But that has not happened yet. Whedon, who will only say that he invested a sum 'below six figures' in the production, 'significantly less than one hour of a TV show', has not yet recouped the money, but believes he will if he can secure a DVD release.

He also intends to make more web-only productions like Dr Horrible, and foresees a new form of artistic community. 'It's a way to tell stories unencumbered. I want to work for a model where the internet could be the whole thing. We could do a feature, we could do a play, we could do whatever we want to do. There is no way this medium can be ignored, especially by me, after years of trying and failing to get TV shows made. The opportunity to put something pure straight out there is too good to miss.'

Although Whedon has the head start of a huge online fan base from his earlier work, other writers and directors may now be tempted to follow his example.

Cynthia Littleton, a TV writer at Variety magazine, said: 'It's proved you can make a big splash, get a lot of attention and be creative and inventive in a somewhat new medium. Put it online and people will come and check you out. There's no question that, going out on a limb like this, Joss has got other producers with the ability to write cheques watching very carefully.'